How to vibrate concrete with a needle vibrator or mechanical trowel

Concrete is a produced from a mixture of cement, gravel, water, sand and additives (setting agents, slowing agents, plasticizers etc.). The preparation is made in the open air and as a result takes in a substantial quantity of air when mixed. This trapped air prevents the concrete from coagulating properly around the gravel by forming air bubbles. The best way to remove unwanted air pockets is to vibrate the concrete once laid and while it is still liquid.

There are three methods of vibration:

• external vibration of the formwork or mould, often used in the manufacture of pre-fabricated concrete

• surface vibration (with a vibrating rule or mechanical trowel)

• internal vibration with a needle vibrator

Formed concrete should present a regular, smooth surface once the formwork is removed. Trapped air bubbles can be a real thorn in your side, so they must be eliminated as soon as the concrete is poured. For a pristine finish, use a needle vibrator.

Formwork structures also need to be internally homogenous - hence the importance of vibration. Reinforced concrete is intended to be highly robust, so it comes top of the list. For instance, a dam can be subjected to extreme forces - so precise mixing calculations and sufficient reinforcement (iron 're-bar') are essential.

Here are just a few reasons:

• air bubbles will rise to the surface, reducing the level of trapped air in the concrete

• the concrete will shrink less, resulting in fewer cracks appearing as the concrete dries

• air pockets and gaps are filled in: porosity is reduced to a minimum to ensure the concrete is impenetrable.

The correct conditions for vibrating concrete

Always avoid over-vibrating since this can cause the gravel component to collect in one mass - which isn't what you want. The duration can be anywhere between 5 seconds and 1 minute, depending on concrete consistency (yielding or firm). Always stop vibrating when:

• the mixture stops settling

• you no longer see air bubbles rising to the surface

• the needle's noise changes (irregular to uniform)

• the surface becomes shiny

A needle vibrator should be completely watertight. They are cylindrical in form, with a domed or conical end for optimum contact with the concrete surface. Inside the cylinder there's an eccentric flyweight which rotates rapidly to produce vibrations, a bit like when something is rattling up and down in the washing machine!

Volume of concrete and vibration rate

Speeds generally range from 3000 to 20000 vibrations per minute (vpm)

most needles are 25-100mm in diameter, although larger can be had for more substantial work. To give you an idea:

• 25-60mm are fine for volumes of up to about 10m³, with high-frequency vibration up to 20000 vpm

• larger diameters from 60-200mm for volumes over 10m³, and lower-frequency vibrations up to 10000 vpm

Operating diameter

Consider the effective diameter of your needle to get an idea of how to place it around the concrete surface - the whole thing needs vibrating evenly:

• spacing between insertion points: 8-9 x needle diameter

• needle diameter: 0.7 x thickness of the concrete surface - not forgetting that it must be inserted between iron reinforcements

Electric or electronic needle vibrators

These portable models are wireless, light and compact. Due to the presence of humidity, double-insulated models are best (European IP 66 classification). Vibrations are generated by an electric motor supplied via a small converter integrated into the device. Here are the important points:

• voltage: 220 V, power 800 W - 3 kW (kilowatts)

• 2 - 6m of cable length between motor and needle

• needle diameter 25 - 65mm

• Vibration rate 10000 - 140000 vpm

• performance up to 8m³/h

• operating diameter given by the formula: spacing = 10 x needle diameter (err on the side of closer spacing, since the waves producing the vibration effect need to spread throughout the concrete)

Pneumatic needle vibrators

Wherever there's pneumatic equipment involved you'll need the services of a compressor. Here are the main points about pneumatic vibrators:

• the most-used type on professional construction sites

• its operation is simple: compressed air drives an eccentric mechanism to produce vibrations

• its action is regulated by modulating airflow

• simple mechanism, no risk of electric shock

• efficient work rate due to high frequency of operation - 8000 to 20000 vpm

Fuel-powered needle vibrators

A fuel engine, either petrol or diesel, transfers rotation via a cable to the needle.

Completely independent of electrical power supply. The semi-rigid transfer cable must be used with care, since it can prove dangerous otherwise.

If you need to finish a flat screed surface, car park or industrial floor, a vibrating rule can be a good option. Depending on the gravel content, this device can vibrate a thickness of up to 12cm.

Electric vibrating rule

A mini electric motor from 0.7 - 0.9 kW is mounted on an aluminium blade 1.5 - 3m wide with an upwards-folded edge. On top are one or two handlebars for steering the device. It produces vibrations at around 1100 vpm. A light, comfortable machine to use (only 25kg), the blades can also be interchanged.

Fuel-powered vibrating rule

Similar to the electric model, with a simple design and powered by a 1 kW fuel motor, this type of vibrator is intended for protracted work (up to 6 consecutive hours). Blades are between 1.2 and 4.9m, and vibration rates go up to 9500 vpm.

The comfortable handle is rubber-coated to minimize the trasfer of vibrations to the operator and allow for better manual control. The handle comes with extensions that can be used to offset it so as to guide the machine from outside of the concreted area.

When finishing larger concrete surfaces, the mechanical trowel or concrete helicopter is the ideal tool. Electric or fuel-powered, These are mainly used on very large surfaces such as in aircraft hangars, supermarket floors or large car parks.

Depending on the shape and type of the interchangeable blades, they can be used to create a smooth finish or to encourage the gravel content of the concrete to sink towards the bottom.

A helicopter must be used at the right point in the process, neither too early nor too late. Veterans will tell you this is when you can still make a footprint without pressing down hard!

Important characteristics of a concrete helicopter

This is a compact machine consisting of a fuel or electric motor driving a single or double rotor which rotates a number of tiltable blades via the action of a hydraulic pump. The blades are contained within a safety casing. A tiller on the end of a long guide pole allows the operator to steer and control the machine. Using it safely and effectively requires a certain amount of practice and training.

You should choose your operating diameter according to the surface area you're working on. For professional use, having access to a range of machines without having to switch rotors will allow you more flexibility.

Available blade types:

• roughing

• finishing

Whatever context you're working in, you'll find what you need to get top-quality results among the various machines on the market.